1. Field of the Invention
This invention relates generally to glasses with good transmittance in the ultraviolet, visible, and infrared regions of the electromagnetic spectrum, while also having low thermal expansion characteristics. These glasses also can be converted, by appropriate heat treatment, to glass ceramics with little or no loss in infrared transmission and also possessing dielectric constants which vary only slightly between about 20.degree. C. and 500.degree. C.
2. Description of the Prior Art
U.S. Pat. No. 3,745,032 pertains to glasses composed primarily of germanium oxide with seven or more other oxides in smaller percentages. The IR transmittance, as illustrated in the graph of FIG. 1, peaks at a wavelength of about 4 microns, decreasing from that point to no transmittance above about 6 microns. Transmittance in the range of 5 to 5.5 microns (a primary range of interest) is a maximum at 5 microns of 60 percent to a maximum at 5.5 microns of about 33 percent, both for Example III.
U.S. Pat. No. 3,769,047 relates to infrared transmitting glasses comprising germania, alumina and lanthana to which are added one or more other oxides to depress the liquidus temperature of the glass. Transmittance of a 2 mm thick specimen is given for six glasses and ranges from 53 to 63 percent at a wavelength of 5 microns and from 23 to 26 percent at a wavelength of 5.5 microns. Coefficients of thermal expansion for these glasses range from 5.80 to 7.37.times.10.sup.-6 /.degree.C.
U.S. Pat. No. 3,982,952 also involves IR transmitting glasses consisting essentially of germania, lanthana, tantala and zinc oxide. Transmittances of these glasses at a wavelength of 5 microns range from 58 to 75 percent, and at 5.5 microns from 32 to 59 percent. Coefficients of thermal expansion range from 6.21 to 7.40.times.10.sup.-6 /.degree.C. As is pointed out in this patent, a moderate coefficient of thermal expansion is a desired characteristic of these glasses to avoid fracture due to heat shock. Also, hardness so as to resist abrasive erosion is desired.